Original practice · fully worked
Original variant: research-vessel repair threshold
A research vessel's repair-cost index X is uniformly distributed from 0 to 24. A reserve pays the amount of X above an unknown activation threshold d and pays zero otherwise. The expected reserve payment is 3. Determine d.
- A
3 - B
9 - C
12 - D
18 - E
21
Answer and calculation
For a uniform index on [0,24], the expected positive excess over d is (24-d)²⁄⁴⁸. Equating this to 3 gives 24-d=12 and hence d=12, so choice C is correct.
Setup
Setup
Represent the reserve payment as the positive part above the unknown threshold.
X∼Uniform(0,24) R=(X−d)+ Model
Model
Integrate the excess payment using the constant uniform density.
E[R]=∫d24(x−d)241dx E[R]=48(24−d)2 Compute
Compute
Set the expected payment equal to its specified value and retain the threshold within the support.
48(24−d)2=3 (24−d)2=144 Answer
Answer
The reserve activates at a cost index of 12.
d=12(C)